It is well known in the art of styrene manufacture to react ethylbenzene (EB) in the presence of steam over a dehydrogenation catalyst, such as iron oxide under dehydrogenation reaction conditions, in order to strip hydrogen from the ethyl group on the benzene ring to form styrene. It is also well known that the dehydrogenation of ethylbenzene requires large amounts of energy, for example, in the form of steam.
Alternate methods for reducing energy consumption (i.e., steam) in processes for producing styrene via dehydrogenation of ethylbenzene have been previously described.
U.S. Pat. No. 4,628,136 to Sardina discloses a dehydrogenation process for producing styrene from ethylbenzene in the presence of steam by recovering heat of condensation normally lost during separation of the various components and using the heat to vaporize an aqueous feed mixture of ethylbenzene and dilution water. Sardina teaches that this obviates the need to use steam to vaporize the liquid ethylbenzene feed.
U.S. Pat. No. 8,163,971 to Wilcox et al. addresses the problem of supplying heat to the system at an overall steam/oil weight ratio of 1.0 or lower. Generally, these ratios would require steam temperature at the outlet of the steam superheater to be increased to 950° C., or even higher. However, superheater temperatures above 927° C. require the use of special and costly metallurgy.
U.S. Pat. No. 8,084,660 to Welch et al. discloses methods for increasing the efficiency and/or expanding the capacity of a dehydrogenation unit by use of at least one direct heating unit. The disclosed methods lower the steam to hydrocarbon ratio of the process to reduce the costs incurred in generating and superheating steam.
U.S. Pat. No. 7,922,980 to Oleksy et al. discloses methods for recovering the heat of condensation from overhead vapor produced during ethylbenzene-to-styrene operations. In this regard, the '980 patent uses the overhead of an EB/SM splitter column to vaporize an azeotropic mixture of ethylbenzene and water.
For economic reasons, however, it is still desirable to lower the steam to hydrocarbon ratio of the process due to the costs incurred in generating and superheating steam. Thus, the inventive methods disclosed herein provide for a reduction of reaction steam/EB ratio while practicing azeotrope heat recovery without resorting to the use of tremendously expensive alloys.